Polarization prevention in electrolysis of sulfide ores



Sept. 4, 1956 N. H. DOLLOFF POLARIZATION PREVENTION IN ELECTROLYSIS OF SULFIDE ORES Filed June 29 1951 INVENTOR. jlipmayflfiaflafl W A v A TTOR/VE Y5 United States Patent POLARIZATION PREVENTION IN ELECTROLYSIS F SULFIDE ORES Norman E. Dolloff, Saratoga, Calif.

Application June 29, 1951, Serial No. 234,245

Claims. (Cl. 204-114) This invention relates to the electrolytic treatment of insoluble metallic sulfides and the separation of the metal and sulfur contents therein. It is applicable to the treatment of mattes, formed as intermediate products in the purification of certain metals and also to the mining of sulfide ores by the conversion of the valuable components thereof to liquid form, where they can be pumped or otherwise drawn to the surface.

Extraction of valuable materials from the ground in liquid form is not new per se. It has been utilized in the Frasch process of sulfur mining, and, to some extent, in the recovery of copper leached from soluble or semisoluble ores by percolating water. Attempts have also been made to purify mattes by electrolysis, but have not proved commercially feasible. Where the sulfides (as mineral or matter) were used as anodes, the desired reactions would take place initially to some extent, but quickly dropped to uneconomical values because of polarization through the liberation of highly insulating elemental sulfur at the anode surface, through which the electrolyte could diffuse very slowly if at all. Increase in applied voltage, necessary to cause material electrolysis, resulted in undesirable and highly inefiicient reactions. Cathodic electrolysis of minerals has been somewhat more successful, but has not proved economically feasible.

Many valuable ores are sulfides. Examples are galena, primarily a lead sulfide but frequently with admixtures of silver and other metallic sulfides, chalcopyrite, a compound of copper and iron sulfide, and chalcocite, a copper sulfide. Many metallic sulfides are extremely insoluble. They are also, as a rule, much more highly conductive of electricity than minerals in general and even more conductive than soil permeated with ground water having its normal small content of ionizable salts. Therefore, if contacts be made with an ore body of this character, having an exposed face covered by an electrolyte, the entire interface between ore and electrolyte can be used as an anode to pass current into the electrolyte and thence to a suitable cathode. The exposed face may be provided either in a shaft or drift, or it may be a bore or well such as are used in petroleum or sulfur mining. Where the latter procedure is used, it is possible to carry out mining operations at depths which it is impracticable for workmen to penetrate, and in situations where ordinary workings would be flooded out.

Objects of the present invention are, therefore, to provide a method of electrolytic treatment of metallic sulfides which can be carried on continuously without detrimental polarization; to provide a method of treatment of sulfide ores whereby not only the metallic but the sulfur content of such ores may be recovered; to provide a method of mining metallic sulfides by the liquid process; to provide a means of mining metal deposits which are otherwise unreachable because of depth or inundation; to provide a method of electrolysis which, with minor modifications only, is applicable to substantially any metallic sulfide; and to provide a method employing aqueous electrolytes which are low in cost and which, in the case of certain ores at least, may be largely derived from the ore itself.

Considered broadly, with respect first to the electrolytic phase of the invention, it comprises the steps of providing an aqueous electrolyte containing an ion which is stable in solution with the ion of the metallic component of the sulfide to be treated, covering the surface of the sulfide with said electrolyte, raising the temperature of said electrolyte above that of the melting point of sulfur and passing current from the sulfide, as an anode, f

into the heated electrolyte. Preferably, the sulfideelectrolyte interface is disposed at a material angle to the metal content can be deposited upon a cathode from p which it may later be peeled or scraped.

The melting point of sulfur, in either of its ordinary solid phases, is above the boiling point of Water at atmospheric pressure. In order to carry out the process of this invention, the boiling point of the electrolyte must be raised to at least the melting point of the alpha-orthorhombic form and usually and preferably above that of the beta-monoclinic form, the first-mentioned form melting at 112.8 C. and the latter at 119 C. Which of these forms the sulfur liberated by the process will takedepends upon factors which cannot generally be predicted, and hence good practice dictates that the temperature of the electrolyte should be raised above the higher of the two values. Two methods of raising the boiling point are readily available; a highly concentrated solu tion may be used as the electrolyte at atmospheric or near atmospheric pressure, or a weaker solution may be used under an absolute pressure of two atmospheres or more. Intermediate values of both concentration and pressure may, of course, be employed. In employing the process in deep mining, particularly by the bore-hole method, this question presents no problem, since a column of Water in excess of about thirty-two feet is suflicient to apply the necessary pressure. In this case, of course, a safety factor must be allowed, in order to prevent a sudden blowout or geyser-like action due to inadvertent rise of temperature above the intended value or a drop in pressure from loss of electrolyte or supernatant liquid by seepage, evaporation, or otherwise. In such mining operations, it is also preferable to provide a diaphragm within the bore to prevent diffusion and convection with consequent loss of heat. The heat may be supplied in various ways; an electric heating element may be used, live steam may be introduced into the electrolyte in situ, in order to start the electrolysis, in many cases the reactions themselves are exothermic and, once started, will carry on without the external application of the heat, and, since the electrolyte has resistance, heat is also supplied by the process itself.

In the drawings:

Fig. 1 is a diagrammatic vertical section of a geological mass including an ore body, together with equipment for applying the method of this invention to the mining of the ore therein; and

Fig. 2 is a plan view of the cathode employed in the equipment of Fig. 1.

In the following description, the process of the invention will be described as applied to two characteristic ores,

Patented Sept. 4, 1956 j place, vi. e.,

s i.e., galena and'chalcopyrite. :The application of the Process toother ores will be apparent, to those skilled in the art, from the descriptions given. Considering first galena, this material consists primarily-of lead sulfide, PbS, usually with a greater or less admixture of some other sulfides, such as thoseof silver,

zinc, :copper, and, to a lesser extent, other metals. Considering only the lead content of the :ores, the anodic reactionaat ery low current densities is V a V .Pb S 2e==Pb++-lS (1) If the electrolyte chosen be sodium chloride, NaCl, the

, lead will go into solution as a chloride, since the latter salt, .while only slightly soluble at room temperatures, is freely soluble in hot water. At extremely low current densities, in .the neighborhood of 0.0001 amp. per cm}, this is substantially the only reaction that ,occurs, and no in the release of the bivalent lead. With such low cur,-

i v, rent'densities, however, the electrolysis is extremely slow and from a commercial point of view it is not feasible.

-.At higher current. densities, a second reaction takes from theelectrolyte. The product is the highly insoluble lead sulfate whichwould of itself cause polarization and tend to :bring the process to a belt, and which is'not readily recoverable.

The extent to which the second reaction takes place depends upon the current density,

. as mentioned above, and eight negative ions are required to react with each molecule of galena. This reaction, therefore, need not take placelto any great extent in order greatly tolower the anode efiiciency. The two reactions toccur together, however, and an increase of s current-density .to 0.01 ampere per square centimeter reduces the anode efiiciency to something in the neighborhood .of 80%, while 'a current density of;0.1 ampkper square centimeter brings it to the neighborhood of 65%: With ver-y'large anode surfaces such as are availablein ore bodies, currents of from 100 to 1,000 'amperes per 7 square meter imply that the limit imposed *upon the process by this factor is not that of current density,,.but of theflcapacity of thzequipment that can economically beemployed. The anode efiiciency varies approximately; inversely as the logarithm of the current density within theranges' which are generally :most economical. While 'there isno'sharp line of demarcation, I prefer to operate at densities of 0.1 ampere per cm. or less. 7 fEven 'taking the latter factor into considerationftllc process would not. be practicallfor galena were the sodium chloride electrolyte tobe employed, owing .to. "polarization 'by the lead sulfate formed. If, however, calciumjchloride be-substituted for the sodium chloride,

the calcium ions react with the liberated sulfate ions to form theonly slightly soluble calcium sulfate, leaving the lead to go into solution 'as the chloride as before. Practical'current densities 0.01 to 0.10 ampere per cmfi, can therefore be used at only slight decrease in the anode efiicieney below that which obtains in extremely low current densities. Any other, calcium saltwhose negativeioniwould combine to form a soluble salt with lead W could, ofcourse, be used in like manner,,e...g., calcium acetate. Similarly, a barium chloride couldjbe used for this purpose. Practically, however, because of .the. low

cost of'calc'iumchloride in comparison with the numerous otherpOs'sibIe materialathe calcium chloride would ordinar'ily be 'the material of choice, particularly for mining "operations. V

The 'elficiency 'of theprocessan'd'the reactions which occur are also afiected bythe concentration of :the various ions in the electrolyte and the cathodic reactionsa As The four equivalentsvof water are, of course, derived the type made familiar by 'itsuse in electric ranges and suming that the solute in the electrolyte, as initially supplied, is calcium chloride, the initial cathodic -reaction1 is the liberation of hydrogen gas. As the concentration of lead ions in solution increases, these eventually migrate to the-cathode and are there deposited, usually as spongy lead.

One method ;by which the invention may be employed in actual mining operations is illustrated diagrammatically in the drawing. The figure illustrates a section through a geologic mass, the major portion of which is country rock consisting largely of silicates and designated by the reference character 1. Intersecting this mass is.

a vein ,3 within which is located'the ore body 15 which it is desired to mine. Ore bodies may be more or less continuous within the vein, audit will be apparent that the larger and more continuous the ore bodies are, the

more economical will be the extraction of their values. by the method of the present invention. -A bore hole 7 is drilled by any of the well known methods, so that it intersects the ore body 5. Obviously, it is desirable it.

the bore can be so produced that it generally follows the vein, so that'there is a possibility of its penetrating ore anywhere along its entire course, but it is' satisfactory if it merely intersects the ore as shown. Ordinary drilling and samplingoperations will show the location of the ore and hencethe position at which the electrolysis can best be carried out. i

The cathode is then lowered into the bore to a position is I surrounded by the ore. The cathode may be nearly. any

type .of conducting body, and suspended on a..cab1e. Preferably, however, it will take somegsuch form .as that illustrated in thefigure. radial fins 9projecting outward from a pipe 11. The material of the cathode is not particularly important, but if it be of iron, it preferably is either copper or lead coated to prevent attack by the electrolyte. This is also true-of both the fins 9 and the pipe 11, insofar as the portion within the ore body is concerned.

Pleferablylhe pipe 11 is insulated ;by' a coating 13 7 Such an insulating applied above the cathode portion. coating is not strictly necessary, due to the verylow voltages used and the natural concentration ofcurrent i'nthe areas of higher conductivity within the ore body. Eventual economy is achieved, however,'by providing the upper portion ofthe pipe 11 with the coating 13, which, since it has to withstand fairly high temperatures may' be made of one of the more heat resistant synthetic rubbers such as thiacoL? A ring 15, mounted at the lower .end of the pipe 11,

carries downwardly projecting columns 17 whichprojectthrough a disc :19 and have mounted, on their lower ends, an electric heating coil .21. This cell may be .of

marketed under the trade name .of Calrod?, but .other A spider .27, is mountedat the top of the 9,. A

bag 29 of canvas or other porous fabric is secured .to' this spider, and to the disc 19 .at the bottom of the piper .It is des'irable but not necessary, that the cathode conductor pipe 11 be surrounded -by an additional pipe :31

which terminates slightly-above the .cathddefistructrire. Adjacent the lower endlof the .pipe 3!. .is ,a.-.dia phragm 33 of such size as nearly to fill the bore. The diaphragm may beef metal or it may be of some fiexiblematerialf such as ,rubber or thiacol, which .will actually make contact with the walls of the bore. The purpose .of this diaphragm isto check convection currents through ,the

bore andconsequentloss of heat from the .cell wherein thereactionistakingplaceu. m

The cathode .conductorpipe 11 .is connected to the neg tive .termi al of a low voltage.D..C. source35. Ihe- This shows a plurality of 7 positive terminal of the source connects to the ore body, preferably through a conductor 37 which is actually driven into the ore. This is not always necessary, but is always desirable. If the anode connection be made through a simple low resistance ground at the surface, actual contact with the anode will almost invariable be by electrolytic conduction, the ground water and its dissolved salts acting as an electrolyte. There will be resulting corrosion and polarization, and hence a large waste of energy at the ground connection. Therefore, although a ground of this character is possible, it greatly reduces the efficiency of the process as a whole and the additional expense of providing a direct contact to the ore body is warranted in practically all cases.

Suitable instrumentation will, of course, be provided for the control of the process. Of these, only ammeter 38 and voltmeter 39 are shown, since these give the primary indications by which the nature of the reactions and the rate at which they are taking place may be judged. Ordinarily, means will be provided for checking the temperature Within the cell, but since such equipment does not directly take part in the process of the invention, it is not shown in the drawing.

The electrolyte is introduced into the cell through either or both of the pipes 11 and 31. If the ore being mined be galena, the calcium chloride electrolyte mentioned will ordinarily be used. The bore above the diaphragm 33 may be filled with water or electrolyte, as desired, to a sufficient height to provide enough pressure to permit the superheating of the electrolyte in the cell.

In operation, the electrolyte is first brought up to the required temperature by means of the heater 21. When this temperature has been reached, the electrolysis is started, the generator 35 supplying an actual voltage across the cell of between two and three volts, although that at the generator terminals will usually be higher. As electrolysis progresses, the metal is liberated on both the cathode fins and the pipe itself in the form of spongy lead. Most of this lead is so slightly adherent to the cathode surface that after it has been built up for a time it falls off and collects in the bottom of the bag 29 as a sludge, as is shown at 41. The sulfur, liberated at the ore surface, also collects in a molten state in the bottom of the bore as shown at 43.

To the extent that the reaction taking place is in accordance with the first reaction given above, the only product liberated at the cathode is lead and no gas is evolved. The oxidation of the sulfur to sulfate, however, results in liberation of hydrogen, which escapes up the bore and can be collected, purified, and utilized.

The other products of the reaction can be collected in various ways. The electrode structure may be withdrawn from time to time and the bag 29 emptied, any adherent lead being scraped or washed off of the cathode surface. Usually, however, the lead sludge can be removed by pumping it out through the inner pipe 11, at the same time forcing electrolyte down through the outer pipe to force the circulation. This also serves to wash spongy lead E of the cathode. The sulfur can be pumped out separately.

It will be noted that the heater 21 may not be a necessary part of the equipment. It is quite possible, for example, to supply the initial heat by the injection of live steam, which can be done through the pipe 11. Once the reaction is started, it may be possible to carry it on rapidly enough and with the evolution of enough heat to make it self-sustaining. The reaction of Equation 1 above, is exothermic. The PR loss in the electrolyte also supplies heat. The total amount of heat necessary to maintain the cell above the melting point of sulfur therefore depends primarily upon the rate at which it is lost. This is smaller than might be expected, for even though the metallic sulfides which conduct electricity are better heat conductors than are most earths and minerals, thermal conduction through them is still relatively poor.

Metals other than lead which may be present in the ore will react in various ways, depending both on their own nature and that of the electrolyte. In the case here considered, silver will be precipitated as the chloride, and can be recovered from the sludge of calcium sulfate which accumulates at the bottom of the bore under the melted sulfur. At the preferred current densities any zinc will normally stay in solution in the electrolyte, as the lead would replace any which might be otherwise deposited on the cathode. those here contemplated, or with a metal such as tin, closer to lead in the electrochemical series, an alloy with the lead could be formed at the cathode and separation by further processing may be necessary.

Replenishment of electrolyte is a necessary concomitant of the electrolysis of galena as here set forth. There will always be some accidental loss, and, moreover, the size of the chamber containing the electrolyte is constantly increasing. If materials, such as silver, which form insoluble chlorides are present, there will be some depletion of chlorine ions in the solution, but with the percentages of silver usually present in galena, the quantity of silver chloride formed will not usually be sufiicient to cause polarization, but it will slough off with the gangue. The chlorine ions in the PbClz are reliberated on the deposition of the lead at the cathode.

If significant amounts of zinc are present, the recovery thereof can be facilitated by causing a slow circulation of the electrolyte from top to bottom of the cathode, pumping thesolution out from the bottom of the bore, precipitating the zinc therefrom, and returning the remainder to the cell.

One property of the process is that certain sulfides which are not of themselves conducting and which hence would not ordinarily be expected to enter the reactions and go into solution do so in fact. Thus the zinc sulfide (Sphalen'te) which frequently occurs in conjunction with galena is decomposed in conjunction with the latter by virtue of a side reaction; apparently an initial replacement of the zinc by lead already in solution, and then decomposition of the lead sulfide as already described. Another side reaction which may take place where pyrite is present is an initial oxidation of the iron from the ferrous to the ferric state, and then secondary oxidation of non-conducting sulfides by the ferric iron.

As the reaction continues, the portion of the bore wherein the electrolysis is taking place will become enlarged as the sulfide ore is corroded. The ores themselves are almost always more or less intermingled with gangue, which is inert and which will drop away into the bottom of the bore. As the chamber within which the reaction is taking place enlarges, more current may be supplied to maintain the anode current density constant, and the voltage drop due to the increase in the length of path from cathode to anode will then increase. If the current be kept constant, the mean cross-section of the current path will increase in approximately the same ratio as the path length. The anode current density will then decrease, with an increase in anode efii ciency as stated above.

If the amount of gangue be relatively large as compared with the sulfide ore, it may not fall away and leave a fresh surface completely exposed but, instead, remain in place as a sort of porous lattice. When this occurs, the efiiciency of the process is somewhat reduced owing to the slower diffusion of the lead ions into the body of the liquid. Under these circumstances, a higher over-all electrical efiiciency can sometimes be attained by applying a pulsating direct current instead of permitting the current to flow continuously. On the other hand, the condition mentioned may result in a greatly enlarged anodic area, the exposed surface being much greater than the mere geometrical circumference of the bore would indicate. This automatically results in a lower current density if the amperage is maintained at a constant value,

At higher current densities than The; treatment of other sulfides by the process of this invention does not, in general, involve the same problemsas does. the treatmentof galena, and while certain other problemsare involved, the treatment may be considerably easier and more economical in various ways.

Typical of both phases of such variance of the process is the treatment of chalcopyrite ores. This material is a sulfide of copper and iron, CuFeSz, which usually occurs more or less associated and intermingled with pyrite, FeSz. employed with galena. In initiating the treatment of this material, the normal choice of electrolyte would be sodium chloride, as being the cheapest and usually most available, although it is perfectly feasible to use sulfuric The equipment used may be the same as that acid or sodium sulfate, magnesium chlorides or sulfates,

ferrous or ferric sulfate, or mixtures of the soluble alkaline or alkaline earth salts, including all of the halides. e The use of sea water or of the water occurring in many a saline lakes .is possible. The reason for this'freedom of choice is that the sulfates of both copper and iron are soluble as'well as the halide salts of these materials. Hence, while the formation of sulfates does take place at the higher current densities, these sulfates do not cause polarization, and the sulfate ions do no harm when they enter the electrolyte and need not be removed by precipitation, as in the case of the electrolysis of galena. As long as the sulfur polarization is preventeththe electrolysis will continue, and the presence of sulfate ions in the solution tends to inhibit the formation of further sulfate ions and thus increase the efficiency of the process.

Starting with any'electrolyte mentioned, the initial re-l action will be the formation of ferrous and cupric ions and elemental sulfur or sulfates by reactions strictly comparable to those already mentioned in connection with galena. At higher voltages, ferric ions may form. With out following the reactions in detail, the f'solution becomes more and more concentrated in sulfate ions. Both copper'and iron will tend to be deposited upon the cathode, 1 but the copper, being higher in the electrochemical series than the iron, will be deposited first and will, in fact, tend to replace the iron, and if the cathode potential be sufiiciently accurately controlled, only the copper will be deposited. ,The copper can therefore be recovered by withdrawing andjscraping the cathode, or by pumping out as a sludge, as in the case of lead If a slow circulation is maintained between the: top andbottom of the cathode, the electrolyte may be withdrawn, the copper content precipitated, and a portion of the ferrous sulfate returned to the top of the cathode to act as the electrolyte to augment the original solution. Under favorable conditions, only water need be added to fill the enlarging bore.

The same general conditions as to the reactions taking place at various current densities. hold with respect to the electrolysis of chalcopyrite as with galena, and low .current densities are therefore more economical. than higher ones. Because'chalcopyriteusually occurs with an admixture of pyrite, however,', an additional effect may enter which also makesit important that the-current density be carefullycontrolled.

In the electrolysisof chalcopyrite-pyrite nuXturesat room temperatures, there is'a marked separation of the two constituents at the anode if'the cell voltages-and current densities are low. Thus, while for lower current densities (0.01 ampjcmfi'orless) the anode efiiciencies, in terms of the copper and iron'content of chalcopyrite,

are comparable to those obtaining in the electrolysis of galena, an increase of one order of magnitude in current densitymay drop the anode efliciency from something over 70% to less than 3% where mixed chalcopyrite and pyrite are present,;and-instead of themetal going; into solution in approximately equal equivalents of cop,-

per; and. iron, fllQQopper drops to a small fraction of'the.

There is evidencethat this separation "at low current densities is due to selective polarization of the pyrite" by sulfur. Therefore, with certain-ores-a higher electrochemical efiiciency can'be attained by alternating the hightemperature electrolysis with lower-temperature electrolysis,'carrying on the latter until the voltage necessary to maintain the desiredcurrent rises to a degree that results in a largeliberation of iron, then depolarizing by raising the temperature and melting oif the sulfur, returning again to the low-temperature operation and so repeating the cycle indefinitely. I

It should be noted that the process as here disclosed yields not only the metallic constituent of the ore but also the sulfur, and that in a form which is either already quite pure or which may readily be purified, frequently by merely settling out the debris sloughed off from the anode surface, or, in any event, by sublimation. An operation which might not be economically feasible from the point of view of the metal values alone may become profitable by reason of the byproduct sulfur.

It will be recognized that the fundamental reactions employed in the method of this invention areflnotof them selves new. They have been employed in the past to a limited degree, but have never achieved commercial success. This is because the polarizationbythe deposit of sulfur or sulfates on the anode surfaces has resulted in the necessity for increasing the electrolyzing voltage to a degree which has not only of itself increased the power requirements, but also hascaused a major portion of the current supplied to go to the oxidation of the sulfur. The primary value of the present invention lies injthe fact that it permits the reactions which have been commercially discarded to be carried on efficiently and economically, in that the polarizing sulfur removes itself, as it were, from'thc field of the reaction and permits it to continue uninhibited. In. substantially all practical applications of the meth0d,-the anode surface will be much more nearly vertical than horizontal, and the molten sulfur will simply drain off, collecting at the bottom of the cell. This will be true Whether the latter is an electrolytic vat, a bore or well, or the bottom of the shaft of a flooded mine. Where this drainage' of sulfur does not'occur by gravity alone, it can easily 'be accomplished by washing with circulating electrolyte,

, brushing, or other mechanicalmeans, but such conditionsoccur so rarely that they are mentioned merely for completeness; a

The twoexamples here given of operation in accordance withthe invention are typical but not necessarily the most favorable ones. in the electrolysis of chalcocite, CuzS, for instance, there is no iron combined with the copper to complicate the reaction, and very high efficiencies can be attained. q j The conditionsfmet within'ore :deposits are too diverse to permit a complete exposition 'ofthe possible variants of theinventionh Such variants, to meet specific cases, shouldbe obvious to those skilled inthe art,

in the light of the examples 'here given. The latter'are intended to be illustrative and not limitations on'the scope of .the 'follo'wing'claims. V Y

I claim: H

1. In an electrolytic process wherein an anode com prising a metallic sulfide is immersed-in an aqueous electrolyte and wherein the liberation of sulfur at said anode tends to cause polarization thereat, the method of substantially preventing such. polarization which com-.

prisesthe step of heating said electrolyte toa temperature above the melting point. of the liberated sulfur.

2. In an electrolytic process wherein an anodev comprising a metallic sulfide is immersed in an aqueous electrolyte and wherein the liberation of sulfur at said anode tends to cause polarization thereat, the method which includes the steps of limiting the current density at said anode to a value of less than 0.1 ampere per square centimeter to cause the major portion of the sulfur of said anode to be liberated in elemental form and heating said electrolyte to a temperature above the melting point of sulfur to permit its ready removal and substantially prevent polarization thereby.

3. In an electrolytic process wherein an anode comprising a metallic sulfide is immersed in an aqueous electrolyte and wherein the liberation of sulfur at said anode tends to cause polarization thereat, the method which comprises the steps of establishing a surface of said anode at a material angle with the horizontal, and heating said electrolyte to a temperature higher than the melting point of the sulfur liberated at said anode, whereby the sulfur so liberated runs off of said surface, thus substantially preventing polarization thereby.

4. In an electrolytic process wherein an anode comprising a metallic sulfide is immersed in an aqueous electrolyte and wherein the liberation of sulfur at said anode tends to cause polarization thereat, the method which comprises the steps of applying sufiicient pressure to said electrolyte to raise the boiling point to a value higher than the melting point of sulfur, and raising the temperature of said electrolyte to a value higher than said melting point to melt the liberated sulfur and so permit its ready removal from the surface of said anode.

5. The method of mining electrically conducting sulfide ores which comprises the steps of exposing a surface of the ore to be mined, covering said surface with an aqueous electrolyte the negative ions whereof are stable in concentrated solution with the metal to be mined, raising the temperature of said electrolyte above the melting point of sulfur, establishing an electrical contact with said ore and passing electric current from said ore as an anode into said electrolyte.

6. The method in accordance with claim which includes the steps of forming a depression beneath the exposed surface of said ore to permit sulfur liberated from said ore to collect in said depression, and pumping said sulfur therefrom in the molten state.

7. The method of mining lead sulfide in accordance with claim 5 wherein said electrolyte contains as a solute a salt of a metal which forms a sulfate of low solubility.

8. The method in accordance with claim 7 wherein the solute in said electrolyte is a halide of an alkalineearth metal.

9. The method of treating an electrically conducting substantially insoluble sulfide of metal which comprises the steps of establishing an electrical contact with said sulfide, covering a surface of said sulfide with an aqueous electrolyte the negative ions whereof are stable in concentrated solution with ions of said metal, passing an electric current from said sulfide as an anode into said electrolyte, and maintaining the temperature of said electrolyte above l12.8 C. to remove polarization from sulfur by melting the same under the electrolyte.

10. The method in accordance with claim 5 of min ing sulfide ores containing copper and iron which cludes the step of electrolytically depositing copper dissolved by said electrolyte While permitting iron salts produced by electrolysis of said ores to accumulate in solution in said electrolyte to continue the electrolytic process.

References Cited in the file of this patent UNITED STATES PATENTS 272,391 Thiollier Feb. 13, 1883 636,321 Craney Nov. 7, 1899 669,442 Frasch Mar. 5, 1901 840,511 Packard Jan. 8, 1907 1,441,063 Christensen Jan. 2, 1923 1,456,798 Hannay May 29, 1923 2,404,206 Arvidson July 16, 1946 OTHER REFERENCES McMillan: Electrometallurgy, 4th ed., 1923, Griffin & Company, London, pp. 289 to 293. 

1. IN AN ELECTROLYTIC PROCESS WHEREIN AN ANODE COMPRISING A METALLIC SULFIDE IS IMMERSED IN AN AQUEOUS ELECTROLYTE AND WHEREIN THE LIBERATION OF SULFUR AT SAID ANODE TENDS TO CAUSE POLARIZATION THEREAT, THE METHOD OF SUBSTANTIALLY PREVENTING SUCH POLARIZATION WHICH COMPRISES THE STEP OF HEATING SAID ELECTROLYTE TO A TEMPERATURE ABOVE THE MELTING POINT OF THE LIBERATED SULFUR. 